Eleven of 48 (23%) patients diagnosed as having lung abscess or empyema presented diagnostic problems in the localization of infected material. All 11 patients were found eventually to have empyemas, all but one of which was complicated by bronchopleural fistulas. Difficulty in distinguishing abscess from empyema on a chest roentgenogram delayed diagnostic and therapeutic thoracentesis from 1 to 12 days. Pleural effusions were noted in all but one of the patients who did not initially have a bronchopleural fistula. In addition, once the bronchopleural fistula became established, the extension of the air-fluid level to the chest wall, the tapered borders of the air-fluid pocket, and the extension of the lesion across fissure lines were noted, in retrospect, to be suggestive of pleural localization. Delay in the evacuation of empyema fluid can lead to chronic complications and increased morbidity. Early identification and treatment of pleural effusions may avoid these diagnostic and therapeutic problems.